1. Field of the Invention
This invention relates to integrated circuit manufacturing and more particularly to determining the roughness of a surface of a target material.
2. Description of the Relevant Art
High quality gate oxides are essential to the manufacture of MOS transistors with stable and reliable operating characteristics. A layer of an electrical insulating material separates a gate electrode of a metal oxide semiconductor (MOS) transistor from an underlying substrate channel region between a source region and a drain region. This insulating layer is commonly made of silicon dioxide (oxide), and the oxide insulating layer is called a gate oxide. A voltage applied to the gate electrode in excess of a minimum "threshold" level attracts enough electrical charges into the channel region to form a conducting path between the source and drain regions.
In order to increase the current sourcing and sinking abilities (i.e., current drives) of MOS transistors, gate oxides are typically made as thin as possible. As the strength of the electric field developed between the gate electrode and the underlying substrate channel region and within the gate oxide is inversely proportional to the thickness of the gate oxide, thinner gate oxides must withstand higher electric field strengths. The maximum electric field strength a gate oxide can withstand before destructively breaking down depends upon the thickness of the gate oxide and the quality of the gate oxide. Thus thinner gate oxides must be of higher quality.
Electrical "oxide rupture" or "rupture voltage" measurements are often performed upon test structures (i.e., MOS capacitor structures) fabricated upon wafers in order to determine gate oxide quality. An oxide layer to be analyzed is formed on an upper surface of a semiconductor substrate, and an electrode is formed on an upper surface of the oxide layer. During rupture voltage testing, a voltage applied to the electrode is continually increased until the oxide is physically destroyed (i.e., "ruptures") and current flows freely from the electrode to the underlying substrate. The maximum applied voltage (i.e., electric field strength) that the oxide can withstand before breakdown is a function of the oxide thickness and quality.
"Pinhole" defects in oxides can cause unacceptably low oxide breakdown voltage levels (i.e., electric field strengths). FIG. 1 is a partial cross-sectional view of an MOS capacitor structure 10. MOS capacitor structure 10 includes an oxide layer 12 formed upon an upper surface of a semiconductor substrate 14, and an electrode 16 formed upon an upper surface of oxide layer 12. Oxide layer 12 is shown to include a pinhole defect 18 in the upper surface of oxide layer 12. Pinhole defect 18 is filled with the conductive material of electrode 16 when electrode 16 is formed. The thickness of oxide layer 12 is reduced in region 20 surrounding pinhole defect 18. During rupture voltage testing, the strength of the electric field between electrode 16 and substrate 14 within oxide layer 12 is greatest in region 20, and oxide layer 12 is more likely to fail first in region 20. The presence of pinhole defect 18 is thus likely to result in a measured breakdown voltage that is lower than expected.
Pinhole defects may be identified in very small "sample" regions of an upper surface of an oxide layer using microscopy. One such method is atomic force microscopy (AFM). AFM allows an area about 10 microns square to be viewed for the presence of pinhole defects. The equipment needed to perform AFM is, however, large and expensive, and studying a sufficient portion of an upper surface of an oxide layer in order to determine the number of pinhole defects present per unit of surface area would be time consuming. Thus AFM cannot be used efficiently in a manufacturing environment subject to volume production.
Oxide surfaces having few pinhole defects tend to be smooth, and oxide surfaces having a large number of pinhole defects per unit of surface area tend to be rough. Thus a measurement of the roughness of the upper surface of an oxide layer reflects the number of pinhole defects per unit of surface area (i.e., the quality of the oxide layer). It would thus be desirable to have a surface roughness measurement technique which is relatively fast and inexpensive, does not involve physical contact with structures on wafers (i.e., non-contact), and also does not result in the destruction of a specimen (i.e., non-destructive). Such a technique could be used to determine the qualities of oxides formed upon product wafers (i.e., wafers expected to yield operational integrated circuits), and would be well suited for use in a high-volume manufacturing environment.
X-ray fluorescence (XRF) spectroscopy techniques are commonly used to determine the elemental compositions of materials. In semiconductor manufacturing applications, a beam of primary X-ray photons is directed at the surface of a semiconductor wafer, and the energy levels (or corresponding wavelengths) of resultant secondary X-ray photons emitted by atoms of elements on and just under the surface of the wafer are measured. Atoms of elements in target materials emit secondary X-ray photons with uniquely characteristic energy levels (or corresponding wavelengths). Thus the elemental compositions of materials on and just under the surface of the wafer may be determined from the measured energy levels (or wavelengths) of emitted secondary X-ray photons. XRF spectroscopy techniques offer rapid, non-contact, non-destructive determination of elemental composition down to trace quantities over a wide range of elements with no sample preparation, and is thus a highly desirable tool for in-line product wafer examination.
FIG. 2 is a side elevation view of a typical XRF spectroscopy apparatus 22. Apparatus 22 includes a high-power X-ray source 24, a sample stage 26, an X-ray detector 28, and a computer system 30. A planar backside surface of a semiconductor wafer 32 is placed on a flat upper surface of a sample stage 26, allowing a planar frontside surface of semiconductor wafer 32 to be subjected to primary X-ray photons during analysis. X-ray source 24 must be a high-power X-ray source in order to produce a relatively large number of X-ray photons per unit time. X-ray source 24 produces a beam of primary X-ray photons 34. The X-ray photons making up primary X-ray beam 34 strike the frontside surface of semiconductor wafer 32 within an exposed region, and have sufficient energy (i.e., sufficiently short wavelengths) to cause atoms of elements of interest located on and just under the surface of semiconductor wafer 32 to emit secondary X-ray photons. A portion of the X-ray photons making up primary X-ray beam 34 and striking the frontside surface of semiconductor wafer 32 are reflected away from the surface, forming a reflected primary X-ray beam 36.
Atoms of elements on and just under the exposed region of the frontside surface of semiconductor wafer 32 absorb a fraction of the incident primary X-ray photons and emit characteristic secondary X-ray photons. FIGS. 3, 4a, and 4b will be used to describe this phenomenon. FIG. 3 is a side elevation view of a primary X-ray photon 40 incident upon a target material 42, resulting in the emission of a secondary X-ray photon 44 by target material 42. X-ray photon absorption and emission occur at the atomic level. XRF spectrometry permits examination of target material 42 from the surface of the target material down to a maximum escape depth 46 of secondary X-ray photons.
FIG. 4a is a representation of an atom 48 of target material 42. In the simple atomic model shown, atom 48 has a nucleus 50 surrounded by electrons 52 at different discrete distances from nucleus 50 called electron shells. A given electron shell has a binding energy level equal to the amount of energy required to remove an electron from the electron shell. The binding energy level of an electron shell is inversely proportional to the distance of the electron shell from the nucleus. The innermost electron shell of an atom is called the K shell, and has the highest binding energy level associated with it. In FIG. 4a, K-shell electron 54 is located in K shell 56.
FIG. 4a also shows primary X-ray photon 40 impacting atom 48 within target material 42. If the energy level of primary X-ray photon 40 (E) is greater than the binding energy level of a K shell 56 (.phi..sub.K), the entire energy of primary X-ray photon 40 is absorbed by atom 48, and one of the electrons in K shell 56 is ejected from atom 48. As depicted in FIG. 4a, K-shell electron 54 is ejected from atom 48 after primary X-ray photon 40 is absorbed by atom 48. K-shell electron 54 is ejected with a kinetic energy of (E-.phi..sub.K).
With a vacancy in K shell 56, atom 48 is energetic and unstable. The most probable stabilization mechanism is the filling of the vacancy in K shell 56 by an electron located in an electron shell with a lower binding energy level. As shown in FIG. 4b, an L-shell electron 58 in L shell 60, farther from nucleus 50 than K shell 56, may fill the vacancy in K shell 56. As L-shell electron 58 fills the vacancy in K shell 56, atom 48 simultaneously emits secondary X-ray photon 44 with energy (.phi..sub.K -.phi..sub.L). where .phi..sub.l is the binding energy level of L shell 60. With a vacancy now in L shell 60, ionized atom 48 is more stable and less energetic. The energy levels (or corresponding wavelengths) of secondary X-ray photons emitted by atoms of elements in substances on and just under the surface of a target material are uniquely characteristic, allowing the elemental compositions of the substances to be determined.
Referring back to FIG. 2, X-ray detector 28 is positioned directly above the exposed region of the frontside surface of semiconductor wafer 30. X-ray detector 28 receives characteristic secondary X-ray photons and scattered primary X-ray photons, and produces an output signal which represents the energy levels of detected X-ray photons. Computer system 30 is coupled to receive and process the output signals produced by X-ray detector 28. An energy range of interest, which includes the energy levels of primary X-ray photons and secondary X-ray photons emitted by expected elements, is divided into several energy subranges. Computer 30 maintains counts of the number of X-ray photons detected within each subrange. After a predetermined exposure time, computer system 30 stops receiving and processing output signals and produces a graph of the counts associated with each subrange.
FIG. 5 is a representative graph of the counts associated with each subrange. Such a graph is basically a histogram representing the frequency distribution of the energy levels of detected X-ray photons, and is called a spectral pattern. Peaks in the frequency distribution (i.e., relatively high numbers of counts) occur at energy levels of scattered primary X-ray photons (E3) and at predominant characteristic emission energy levels (E1 and E2) of atoms of elements located on and just under the frontside surface of semiconductor wafer 32 within the exposed region. The peak in the frequency distribution occurring at energy levels of scattered primary X-ray photons (E3) is basically ignored. Energy levels E1 and E2 associated with the other two peaks in the frequency distribution are used to identify the one or more elements present on and just under the exposed region of the frontside surface of semiconductor wafer 32 according to well-known XRF methods.
A primary X-ray photon incident upon a target material may be absorbed or scattered. Characteristic secondary X-ray photons are emitted only when primary X-ray photons are absorbed. Primary X-ray photons may loose energy when scattered by atoms of the target material. Such scattered primary X-ray photons which reach the X-ray detector of an XRF instrument create an unwanted background intensity level which secondary X-ray photons must exceed in order to be discerned. Thus the smallest amount of an element which may be detected in a sample using an XRF instrument is largely determined by the background intensity level at the energy level (or corresponding wavelength) associated with characteristic secondary X-ray photons emitted by that element. The sensitivity of an XRF instrument is thus largely dependent upon the background intensity level, and the sensitivity of an XRF instrument may be improved by reducing the amount of scattered primary X-ray photons reaching the detector.
Total reflection X-ray fluorescence (TXRF) is an XRF technique ideally suited for the examination of semiconductor wafers and other materials with substantially planar surfaces. In TXRF, an angle of incidence formed between an incident primary X-ray beam and a substantially planar sample surface is very small, typically less than 0.2 degree. At such small angles of incidence, almost all of the primary X-ray photons striking the sample surface are reflected away from the surface. Primary X-ray photons are also reflected away from an X-ray detector positioned over the region of the wafer surface where the primary X-ray beam impacts the wafer surface. The number of scattered primary X-ray photons reaching the detector is thus significantly reduced. As a result, surface analysis instruments employing TXRF techniques have reduced background intensity levels over other types of XRF analysis instruments. Smaller quantities of elements may be detected with TXRF instruments due to their greater sensitivities.
It would thus be desirable to have a surface roughness measurement technique based upon the scattering of X-ray photons from the surface of a target material. Such a surface roughness measurement technique would be relatively fast and inexpensive, would be non-contact, and would also be non-destructive. Such a surface roughness measurement technique could be used to determine the qualities of oxides formed upon product wafers, and would be well suited for use in high-volume manufacturing environments.